Maybe no. Outook/Exchange is quite a nice piece of Groupware, and there are some features still lacking on default Groupware for NethServer (WebTop), just like… desktop extensible client.
But… it’s worth the try, IMO.
And you can try “for free”, connecting the testing machine to your current AD.
Yes. Read the documentation 
Not directly, at least two options:
- imapsync
- PST import (which is from outlook not Exchange)
(pop3 is not really an option…)
Anyway, check the documentation.
I definitely recommend to use NethServer as DHCP Server
For VPN you have… more options.
- You need an AD-integrated account for VPN? Use NethServer and create an integrated setup, with the server acting as server (AD, Mail, GroupWare, fileserver, DNS) and gateway (with VPN endpoint).
- You don’t need AD-integrated account for VPN? You can install a second NethServer for gateway use only, splitting load on multiple hardware (if not virtualized) and having a separated set of credentials for VPN access. This will lead to a bit more complicated content filtering.
(Both options allow only OpenVPN based tunnels for RoadWarriors)
- You can choose a separate appliance for gateway and VPN endpoint, like a commercial hardware firewall. This will lead to a lower energy consumption and a separated management, if necessary; this might lead also to a more flexible VPN solution (L2TP or IpSec, at least, for roadwarriors), with the risk/possibility to pay licenses for every user should use the VPN connection.
You don’t give much more for… help you making a decision.
One of the best options using NethServer is that you could use a full featured install without time or product limits for evaluation.
Of course, there are other distros/products that allow something similar.